The present invention relates to the field of radiological imaging with X-rays, which allows for example the organs of a patient to be viewed. The invention more particularly relates to the processing of fluoroscopic images for attenuating the noise of the fluoroscopic images to make them clearer; nevertheless, the invention could also be applied, in general, to the processing of any image, and in particular, a radiographic image.
In the field of medical imaging, it is well known to utilize fluoroscopic images to guide surgical instruments during a surgical procedure. These fluoroscopic images are acquired by an imaging apparatus comprising of means for providing a radiation source, such as X-rays, positioned opposite means for receiving an image and/or detecting the radiation, the means for providing a radiation source and the image receiver and/or detector being capable of driven in rotation about at least one axis, usually three axes, means for control, means for acquisition, means for image visualisation and means for command. The patient is positioned on a table to be moved in the three possible translations associated with a given space, that is, longitudinally, laterally and vertically, such that the part of the body of the patient to be examined and/or treated extends between the radiation source and the image receiver. This mobility of the table and of the radiation source and of the image receiver allows a practitioner to acquire the images for any part of the body of a patient lying on the table. Therefore, it is usual to utilize fluoroscopic images in two dimensions obtained by irradiation of the patient using low doses of radiation, a contrast agent preferably being injected previously, during intervention to guide the instrument in the organ of the patient to be treated. The information associated with these fluoroscopic images can be, introduced to images reconstructed in three dimensions for improving the guidance of surgical instruments. Alternatively, images acquired in three dimensions can be projected onto the fluoroscopic images acquired in two dimensions during the intervention.
Contrary to radiographic images acquired by the imaging apparatus emitting radiation in strong doses providing images of good quality with a low noise level, that is, presenting a high signal to noise ratio, the fluoroscopic images obtained with lower doses of radiation comprise a higher noise level, that is, presenting a low signal to noise ratio, and are thus of inferior quality, which is likely to disturb the course of surgical intervention. In effect, the noise registered by the radiation detector and appearing on the images is of quantum origin and depends on the square root of the number of photons detected per pixel. When the dose of radiation is reduced, the noise decreases less rapidly than the dose, such that the noise to signal ratio grows. In addition to the quantum noise the basic movement associated especially with the respiration of the patient is added to the displacement of the surgical instruments and to the movements of the table on which the patient is placed.
In order to eliminate the noise from the images acquired, a time filter could be applied in the hypothesis of immobile images; nevertheless, since the images acquired are mobile in fluoroscopy, application of a simple time filter is translated by a blurred movement and/or loss of contrast of the mobile objects, apart from a noise spike.
A process for treating images in fluoroscopy comprises applying a compensated movement filter. The majority of filters of the prior art utilize a distinction criterion between a variation due to noise and a variation due to movement; nevertheless, stopping filtering causes the reappearance of the noise, which is translated on the images by streaks of noise behind the mobile objects.
In order to rectify these disadvantages, processes for treating a sequence fluoroscopic of images improving the quality of the visualized images have already been conceived. This is the case, for example, in FR 2 790 123, which describes a process in which, for each acquired current image, the moving of the current image is determined relative to the preceding image acquired in the acquisition plane of the images, a preceding offset filtered image is shaped by spatially moving the preceding filtered image, and the current filtered image is shaped by the weighted average between the current acquired image and the preceding offset filtered image. This type of process has the drawback of not yielding satisfying results. In fact, the fluoroscopic images appear as superposition of layers of transparent images such that it is impossible to clearly identify the pixels and the physical objects. Thus, even if the global movement of the image sequence can be determined, the different layers of the objects cannot be separated for filtering them time-wise independently of one another, resulting in loss of contrast of the image and a streak of noise behind the mobile objects.